Micro-scale analysis of intracellular contents, such as nucleic acids and proteins, is gaining importance in biology. Other than enabling minimized analytical and cell biology profiling of processes at the cellular level, microfluidics is also finding new applications relating to micro-culturing of cells for high throughput screening and biological research. The prokaryotic Gram-negative Escherichia coli bacterium and the eukaryotic Pichia pastoris yeast are microbial host cells extensively used for screening of clones from genomic libraries and heterologous protein expression. They both allow the parallel functional expression of multiple proteins in microplate assays, which may be made amendable to micro-scale analysis. However, before the micro-scale analysis can be carried out, an effective microfluidic cell lysis for the release of active intracellular contents needs to be achieved.
In microfluidics, cell lysis may be accomplished by chemical, thermal, electrical, or mechanical means. Of these, chemical lysis with lytic agents and thermal lysis with heat frequently lead to the denaturation of proteins or interfere with subsequent assays. Furthermore, chemical lysis has the added disadvantages of requiring wet chemical storage and intensive mixing, which may add complexity in a microfluidic setting. Although electrical cell lysis may have the advantages of being reagent-less and quick, the application of a direct current at elevated voltage may lead to water hydrolysis, undesirable localized heating, and/or denaturation of proteins. Mechanical lysis, which may involve the generation of high shear through the application of high pressure, rapid agitation, or sonication, often needs intensive cooling to remove the heat produced by the dissipation of the mechanical energy.
Despite the disadvantages of undesirable heating, sonication has been widely used in lab-scale settings to attain mechanical lysis of cells. The basic principle of sonication is to generate mechanical shear stress by oscillating cavitation bubbles using an ultrasound field. In bulk medium, this typically involves the application of a bench-top vibrating probe directly into the liquid. The rapid movement of the probe tip creates a series of rapidly collapsing cavitation bubbles that break apart cells. This process may inefficient and some energy may be lost as heat.